Cast wall construction wherein plaster slabs are formed in place on an underlying support structure are well known. As the plaster slabs are subjected to thermal expansion and contraction or to movement of the underlying support structure, such as by wind loading or settling, the plaster slabs tend to crack. It is well known that such cracking can be minimized by dividing the cast wall into a number of separate sections or slabs, as by the use of a channel or divider strip. In addition, in order to maintain constant thickness of the plaster slab during the casting process, it is helpful to provide a screeding surface.
A typical plaster screed expansion strip is shown in U.S. Pat. No. 3,667,174. This expansion strip comprises an elongated extrusion having a front wall, a pair of opposing side walls extending rearwardly of the front wall, a pair of lateral anchors extending outwardly of the side walls, and a rear wall including a pair of planar flanges for securing the expansion joint to the underlying support surface. The front wall is attached to the opposing side walls only by a thin web of plastic to form a "tear strip", whereby the front wall can be removed after the wall construction is complete. The rear wall is arched inwardly of the two side walls, this arched section being deformable in response to expansion forces exerted against the side walls by the adjacent plaster slabs. The front wall is substantially flush with the edges of the side walls, forming a screeding surface to serve as a guide for the thickness of the plaster slabs cast adjacent to the expansion joint.
A major drawback with the design disclosed in U.S. Pat. No. 3,667,174 is that the inwardly arched rear wall section is interposed between the side walls. Thus, as the adjacent plaster slabs expand and exert lateral forces against the side walls, the expansion strip can never fully contract until the side walls touch one another because the inwardly arched wall is always interposed therebetween. Accordingly, the minimum width to which the expansion strip can contract is limited to the combined thickness of the inwardly arched walls. The extent of this limitation can best be seen when viewed in the light of the dimensions of a typical expansion joint. Plaster screeding expansion joints similar to the aforementioned U.S. Pat. No. 3,667,174 are typically used to form a 0.25 inch joint. The inwardly arched walls are normally 0.025 inches thick, and the combined thickness of both inwardly arched walls is thus 0.050 inches. Accordingly, the minimum width to which the aforementioned joint can contract is 0.050 inches, which is 20% less than the total contraction which would be possible if the side walls were not prevented from touching one another by the inwardly arched rear wall. In other words, if the expansion joint could fully contract until the side walls touched one another, a joint 20% narrower could absorb the same thermal expansion, or 20% fewer joints of the same width would have to be installed to absorb the same thermal stresses.
Accordingly, there is a need to provide a thermal expansion strip for use in cast plaster wall construction which provides increased movement for improved absorption of thermal stresses imposed by adjacent plaster slabs, whereby either fewer expansion strips or narrower joints can be used to absorb thermal expansion stresses.
Another problem associated with the prior art plaster screed expansion joint as disclosed in the aforementioned U.S. Pat. No. 3,667,174 is that a pair of V-grooves are formed between the outer edges of the front wall and the inner edges of the side walls. During the process of finishing the plaster slabs adjoining the expansion joint, it is not uncommon for plaster to be splashed over the forward edges of the side walls and fill the V-groove. When this occurs, a residue of plaster remains on the face of the expansion joint when the forward wall or tear strips is removed. This residue presents an unsightly appearance and requires additional labor to remove from the expansion joint after the tear strip is disengaged.
Accordingly, there is a need to provide a plaster screed expansion joint which resists the accumulation of residue on its outer face during the screeding process.